Efficient Energy Storage Technology for Photovoltaic System

Hoda Akbari a, Maria C. Browne a, Anita Ortega a, Mingjun Huang b, Neil J. Hewitt b, Brian Nortonc, Sarah J. McCormack a*

a Department of Civil, Structural and Environmental Engineering, Trinity College, Dublin 2, Ireland

b School of Architecture and the Built Environment, Built Environment Research Institute, Ulster University, Belfast, UK.

c Dublin Energy Lab., Dublin Institute of Technology, Grangegorman, Dublin 7, Ireland.

*Corresponding author: mccorms1@tcd.ie

Abstract

For photovoltaic (PV) systems to become fully integrated in society, efficient, cost-effective

energy storage systems must be utilized together with intelligent demandside management. As

the global solar photovoltaic market reaches 76 GW, increasing PV self-consumption will

become important to achieve integrate more PV power in the power system. The paper reviews

current energy storage systems for PV. It encompasses (i) electrical energy storage systems

(EES); (ii) thermal energy storage (TES) systems and (iii) integration of PV-energy storage in

smart buildings and (iv) examines the potential and role of energy storage for PV in the context

of future energy developments.

Keywords: Photovoltaic; Phase Change Material (PCM); Thermal Energy Storage (TES);

Concentration; Solar electrical system; Building integration

1. Introduction

Over the past decade, global installed capacity of solar photovoltaic (PV) has dramatically in-

creased as part of a shift from fossil fuels and a move toward reliable, clean, efficient and sus-

tainable fuels (Kousksou, Bruel, Jamil, El Rhafiki, & Zeraouli, 2014; Santoyo-Castelazo & Aza-

pagic, 2014). Energy storage is essential for PV technology to become completely reliable as a

primary source of energy. It is necessary to store PV electricity when an excess is produced to

be released when it is required. Energy storage can help networks withstand peaks in demand

allowing transmission and distribution to operate at full capacity. In terms of shorter periods of

storage, it can be effective for smoothing out small peaks and distortion in voltage (Hadji-

paschalis, Poullikkas, & Efthimiou, 2009).

1

Energy storage technologies can be classified as electrical, thermal and mechanical (Baker,

2008; Ibrahim, Ilinca, & Perron, 2008; Makarov et al., 2008; Schoenung & Hassenzahl, 2001).

Electrical Energy Storage (EES) technologies include:

Supercapacitors (electrochemical capacitors): Electrochemical capacitors sometimes

referred to as “electric double-layer” capacitors and appear under trade names like

“Supercapacitor” or “Ultracapacitor.” The phrase “double-layer” refers to their

physically storing electrical charge at a surface-electrolyte interface of high-surface-

area carbon electrodes.

Electrochemical systems, such as embracing batteries and flow cells;

Pumped hydro: creating large-scale reservoirs of energy with water.

Compressed air energy storage (CAES): utilizing compressed air to create a potent

energy reserve.

Flywheels: mechanical devices that harness rotational energy to deliver instantaneous

electricity.

EES can increase overall system efficiency, improve system performance and reliability, reduce

the cost for better economics, minimize environmental pollution and reduce CO2 emissions

(Balecom et al, 2015). AV can also, via resistance heating, supply a Thermal Energy Storage

TES system which has similar steps in many applications. First the energy is supplied to the

TES systems (charging), and then it will be stored (storage) and finally it will be removed from

the TES for a later use (discharging) (Cabeza, 2012; Dinçer & Rosen, 2010; Mehling & Cabeza,

2008). There are three different types of thermal energy storage:

Thermochemical energy storage;

Sensible heat storage;

Latent heat storage.

The intended end-use determines the most appropriate energy storage medium for PV generated

electricity as shown in Figure 1. For both AC and DC electrical end-uses batteries are suitable.

However if the end-use is heat then direct conversion of the electrical output to heat would be

an efficient option. For other water pumps to continue to supply well water at night it obviates

the need to include unnecessary electrical storage when the pumped water is is stored during the

day for nightime use (Oden et al, 2006, Bakelli et al, 2011). Such water storage is usually

placed at a height tha can provide sufficient pressure to achieve an adequate discharge rate. This

can potentially be used for small scale hydropower (Manolakos et al, 2004, Ma et al, 2015)

2

Figure 1. PV Energy Storage Systems

2. Energy Storage for PV

2.1. Electrical Energy Storage (EES)

The conjunction of PV systems with storage batteries allows a further increase of self-

consumed PV electricity. With a battery system, the excess PV electricity during the day is

buffered and later used at night. In this way, households equipped with a PV battery system can

reduce the energy drawn from the grid and therefore increase their self-sufficiency (Weniger,

Tjaden, & Quaschning, 2014), therefore PV battery systems of various sizes could reduce the

dependence of residential customers on the central grid and their impact on CO2 emissions.

2.1.1.Challenge of using EES for PV

Electrical Energy Storage (EES) refers to a process of converting electrical energy into a form

that can be stored for converting back to electrical energy when needed. Such a process enables

electricity to be produced at times of either low demand, low generation cost or from

intermittent energy sources and to be used at times of high demand, high generation cost or

when no other generation means is unavailable (Ibrahim et al., 2012). From Fig. 2 it can be seen

how storage is charged from baseload generation plant at early hours in the morning and late

hours in the night. This storage energy is used to counter demand in peak hours (around 6 pm).

3

In addition, the storage energy between 6 am and 6 pm maintain frequency and voltage by

balancing supply and demand.

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Fig. 1. Fundamental idea of the energy storage (Ibrahim et al., 2012)

EES can be divided in a range of categories, including mechanical, thermal and chemical

storage. Each of these broad categories has a unique set of parameters to measure cost and

performance. Table 1 summarizes the technical characteristics of different EES technologies.

Table 1. Comparison of technical characteristics of EES (Ibrahim et al., 2012).Storage Technolo-

gyPower rating (MW)

Discharge time Capital cost (USD/KW)

Efficiency (%)

SMES 0.1-10 ms to 8s 200-500 80-90Flywheels 0-10 ms to 40s 250-450 90-95Capacitor 0-0.05 ms to 60s 200-500 60-75

Supercapacitor 0-0.3 ms to 60s 100-500 90-100Batt-ZEBRA 0-0.3 seconds to hours 150-300 85-90

Batt-NaS 0.05-50 seconds to hours 600-3000 75-90Batt-Lead acid 0-20 seconds to hours 300-800 70-80

Batt-NiCd 0-40 seconds to hours 500-1500 60-70Flow-ZnBr 0.05-2 seconds to 10 hours 600-2500 65-75Flow-VRB 0.03-7 seconds to 10 hours 600-2500 75-90Flow-PSB 1.0-15 seconds to 10 hours 600-2500 60-70Metal-air 0-0.01 seconds to 24 hours and more 100-250 30-50Fuel cells 0-50 seconds to 24 hours and more 20-50

Batt-Li-ion 0-0.1 minutes to hours 1200-4000 85-100AL-TES 0-5 1h to 8 hours 40-60

CES 0.1-300 1h to 8 hours 200-300 40-50Solar fuel 0-10 1h to 24 hours and more 20-30HT-TES 0-60 1h to 24 hours and more 30-60CAES 5-300 1h to 24 hours and more 400-1350 70-80PHS 100-5000 1h to 24 hours and more 600-2000 70-85

5

Distributed solar PV-battery systems are installed for many reasons, these can be altruistic

rather an economic and/or relate to achieving local security of supply. PV-battery systems can

immediately provide lower electricity bills where favorable policies, including net metering,

feed-in tariffs, tax credits, and particular rate structures are in place.

PV-battery systems can confer societal benefits, particularly the reduction of CO2 emissions.

Solar PV reduces system level CO2 emissions if it offsets generation from fossil fuels. CO2

reduction benefits are an important motivator for many customers installing PV systems, even

though the benefits accrue to society at large. In addition, PV-battery systems can provide grid-

level benefits that improve the overall efficiency of the electricity grid and reduce system-level

costs (Hanser, et al, 2017).

Benefits of PV-battery systems include the ability of pv generated electricity to be used to:

offset generation from more expensive generators;

reduce congestion on transmission and distribution systems;

stabilize local electricity flow;

control local voltage fluctuations; and

enable transmission and distribution system upgrades to be avoided or deferred.

2.1.2. Types of Electrical Energy Storage for PV

A battery which stores electricity in the form of chemical energy is comprised of one or more

electrochemical cells where each cell consists of a liquid, paste, or solid electrolyte together

with a positive electrode (anode) and a negative electrode (cathode). During discharge,

electrochemical reactions occur at the two electrodes generating a flow of electrons through an

external circuit. The reactions are reversible, allowing the battery to be recharged by applying

an external voltage across the electrodes (Chen et al., 2009).

Currently various batteries are used for the application with pv systems

Flow batteries (ZnBr, VRB and PSB): batteries where the energy is stored directly in

the electrolyte solution for longer cycle life, and rapid response times.

Sodium–sulfur batteries (NaS):

Lithium–ion batteries (Li–ion): Li-ion batteries have been deployed in a wide range of

energy-storage applications, ranging from energy-type batteries of a few kilowatt-hours

in residential systems with rooftop photovoltaic arrays to multi-megawatt containerized

batteries for the provision of grid ancillary services.

Nickel–cadmium batteries (Ni–Cd): can provide long life and reliable service.

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Lead–acid batteries: provide a cost-competitive and proven energy storage but have

relatively limited cycle life, low-energy density and a resulting large footprint (Baker,

2008).

Metal–air batteries: consists of the anode made from pure metal and the cathode

connected to a supply of air (International Electrotechnical Commission and

Commission, 2011).

Fig. 3, shows applications of energy storage systems in accordance with discharge time and

rated power. Table 2, shows a resume of different battery storage technologies system installed

and Table 3, shows batteries storage for utility transmission and distribution grid support.

Fig. 3. Application of energy storage systems in terms of discharge time and rated power (Toledo, Oliveira Filho, & Diniz, 2010).

Table 2. Battery technologies characteristics and commercial units used in electric utilities (Divya & Østergaard, 2009).

Battery Type Largest capacity Efficiency (%)

Cost (euro/KWh)

life span (cy-cles)

Lead acid (flooded type) 10MW/40MWh 72-78 50-150 1000-2000Lead acid (valve regulated) 300KW/580KWh 72-78 50-150 200-300Nickel Cadmium (NiCd) 27MW/6.75-

MWh72-78 200-600 3000

Sodium Sulphur (NaS) 9.6MW/64MWh 89 2500

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Lithium ion 100 700-1000 3000Vanadium redox (VRB) 1.5MW/1.5MWh 85 360-1000 10000Zinc Bromine 1MW/4MWh 75 360-1000Metal air 50 50-200 100Regenerative fuel cell (PSB)

15MW/120MWh 75 360-1000

Table 3. Energy storage for utility transmission and distribution grid support (Dunn, Kamath, & Tarascon, 2011).

Technolo-gy

Capacity (MWh) Power (MW)

Duration (Hours)

Effiiency (%)

Total cost (USD/KWh)

CAES 250 50 5 1950-2150Pb-acid 3.2-48 1.0-12 3.2-4 75-90 2000-4600

NaS 7.2 1 7.2 75 3200-4000Zn/Br flow 5.0-50 1.0-10 5 60-65 1670-2015

V redox 4.0-40 1.0-10 4 65-70 3000-3310FeCr flow 4 1 4 75 1200-1600

Zn air 5.4 1 5.4 75 1750-1900Li-iom 4.0-24 1.0-10 2.0-4 90-94 1800-4100

Lead acid batteries, invented in 1859, are the oldest and most widely used rechargeable

electrochemical devices. A lead acid battery consists of electrodes of lead metal and lead oxide

in an electrolyte of about 37% sulphuric acid. In the discharged state both electrodes turn into

lead sulphate and the electrolyte loses its dissolved sulphuric acid and becomes primarily water.

Lead acid battery has a low cost ($300–600/kWh), and a high reliability and efficiency (70–

90%). It is a popular storage choice for power quality, UPS and some spinning reserve

applications. Its application for energy management, however, has been very limited due to its

short cycle life (500–1000 cycles) and low energy density (30–50 Wh/kg) due to the inherent

high density of lead. To overcome the poor low temperature performance of Lead acid batteries

requires a thermal management system (Chen et al., 2009).

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NiCd batteries contain a nickel hydroxide positive electrode plate, a cadmium hydroxide

negative electrode plate, a separator, and an alkaline electrolyte. NiCd batteries usually have a

metal case with a sealing plate equipped with a self-sealing safety valve. The positive and

negative electrode plates, isolated from each other by the separator, are rolled in a spiral shape

inside a case. NiCd batteries have a high energy density (50–75 Wh/kg), a robust reliability and

very low maintenance requirements, but relatively short cycle life (2000–2500). These

advantages over lead acid batteries make them favored for use in power tools, portable devices,

emergency lighting, UPS, telecoms, and generator starting. The main drawbacks of NiCd are:

(i) the batteries have a relatively high cost due to the expensive manufacturing process and (ii)

cadmium is a toxic heavy metal hence posing issues associated with the disposal of NiCd

batteries (Dunn et al., 2011b).

Sodium-Sulfur batteries consist of a molten sulfur positive electrode and a molten sodium

negative electrode separated by a sodium beta alumina ceramic electrolyte as it is shown in Fig.

4. The electrolyte permits that only positive sodium ions pass to combine with sodium for the

formation of sodium polysulfide. During the discharge process, positive sodium ions flow

through the electrolyte and electrons pass through an external circuit generating a voltage of

roughly 2 V. This process is reversible; and by using the charge, sodium ions are liberated from

the sodium polysulfide and return through the electrolyte to recombine with the sodium element

(Toledo et al., 2010). Characteristics of NaS batteries include:

expected operational duration of 15 years considering 2500 cycles (charge and

discharge) for a 100% depth of discharge (DOD), 4500 cycles for a 90% DOD or 6500

cycles for a 65% DOD;

output voltage (CC) of 64 or 128 V for peak-shaving application modules and 640 V for

energy quality application modules power of 50 kW;

energy of 430 kWJ for application in peak-shaving and 360 kWh for application in

energy quality; rapid response time, complete discharge in 1 ms if necessary;

high energy density, three to five times that of a lead–acid battery;

unaffected by ambient temperatures (operates from 290–360 8 C);

allows for remote operation and monitoring with minimal maintenance;

no emissions or vibrations, low noise compared to other energy conversion systems

98% of the material utilized in the NaS batteries can be recycled, only the sodium

cannot be reused.

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Fig. 4. Schematic of the NaS battery (Dunn, Kamath, & Tarascon, 2011)

In lithium-ion batteries, the cathode is a lithiated metal oxide and the anode is made of graphitic

carbon with a layering structure. The electrolyte is made up of lithium salts dissolved in organic

carbonates. When the battery is charged, the lithium atoms in the cathode become ions and

migrate through the electrolyte toward the carbon anode where they combine with external

electrons and are deposited between the carbon layers as lithium atoms. This process is reversed

during the discharge process. The efficiency of Li-ion batteries is almost 100% another

important advantage over other batteries. Although Li-ion batteries take over 50% of the small

portable devices market, the challenge for making large-scale Li-ion batteries is their high cost

(>$600/kWh) due to special packaging and internal overcharge protection circuits (Chen et al.,

2009).

Metal-Air batteries are the most compact and, potentially, the least expensive batteries (Chen et

al., 2009). Among the various metal air battery chemical couples, the lithium air battery is most

attractive since its theoretical specific energy excluding oxygen is 11.14 kWh/kg, corresponding

to about 100 times more than other battery types and even greater than petrol (10.15 kWh/kg).

However, the high reactivity of lithium with air and humidity can cause fire, which is a high

safety risk. Currently only a zinc air battery with a theoretical specific energy excluding oxygen

of 1.35 kWh/kg is technically feasible (International Electrotechnical Commission and Commis-

sion, 2011). Zinc air batteries have some properties of fuel cells and conventional batteries: the

zinc is the fuel, the reaction rate can be controlled by varying air flow, and oxidized zinc/elec -

trolyte paste can be replaced with fresh paste (Chen et al., 2009). A schematic of metal air bat -

tery is can been seen in Fig. 5.

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Fig. 5. Schematic of the metal air battery (Chen et al., 2009).

A flow battery is a form of a battery in which the electrolyte contains one or more dissolved

electroactive species flowing through a power cell/reactor in which the chemical energy is con-

verted to electricity. Additional electrolyte is stored externally, generally in tanks, and is usually

pumped through the cell of the reactor as is shown in Fig. 6. The reaction is reversible allowing

the battery to be charged, discharged and recharged. In contrast to conventional batteries, flow

batteries store energy in the electrolyte solutions. The power and energy ratings are independent

of the storage capacity determined by the quantity of electrolyte used and the power rating by

the active area of the cell stack. Flow batteries can release energy continuously at a high rate of

discharge for up to 10 hours (Dunn et al., 2011b).

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Fig. 6. Schematic of flow battery (Chen et al., 2009).

Toledo et al. (2010) found that a photovoltaic system with a NaS battery storage system enables

economically viable connection to the energy grid. Having a long useful life NaS batteries have

high efficiency in relation to other batteries, thus requiring a smaller space for installation. The

installation of these systems in densely, interconnected to buildings, renders grid transmission

lines unnecessary. Application of photovoltaic systems with NaS batteries for peak-shaving can

be used by utilities to increase reliability of their systems (Toledo et al, 2010).

Hoppmann et al. (2014) summarized different battery storage for residential solar photovoltaic

systems as shown in Table 4.

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Table 4. Scope of previous studies investigating battery storage with PV system (Hoppmann et al, 2014)Study Lead Acid Sodium

SulfurLithium Ion

Nicken Cadmium

Vanadium Redox Flow

(Askari & Ameri, 2009) √

(Avril, Arnaud, Floren-tin, & Vinard, 2010)

√ √

(Battke, Schmidt, Grosspietsch, & Hoff-mann, 2013)

√ √ √ √

(M. Braun, K. Büden-bender, D. Magnor, 2009)

√

(Celik, Muneer, & Cla-rke, 2007)

√

(Colmenar-Santos, Campíñez-Romero, Pé-rez-Molina, & Castro-Gil, 2012)

√

(Jallouli & Krichen, 2012)

√

(Kaldellis, Zafirakis, Kaldelli, & Kavadias, 2009)

√ √

(Kolhe, Kolhe, & Joshi, 2002)

√

(Lazou & Papatsoris, 2000)

√

(Li, Zhu, Cao, Sui, & Hu, 2009)

√

(Liu, Rasul, Amanullah, & Khan, 2012)

√

(Wissem, Gueorgui, & Hédi, 2012)

√

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For online energy management (OEM) for a lithium-ion battery bank used in PV-based systems

to meet the load demand, the capacity fade of the lithium-ion battery is influenced by

charge/discharge currents, DOD, temperature and cycle number (Feng et al, 2015).

2.1.3.Advantages of EES for PV

Battery storage is an effective means for reducing the intermittency of electricity generated by

solar photovoltaic (PV) systems (Hoppmann et al., 2014) to improve the load factor,

considering supply side management, and the offer of backup energy, for demand side

management. PV systems have often been installed to feed the generated electricity into the

electricity grid, which was remunerated with feed-in tariffs. However, this changed in the

beginning of 2012, when the feed-in tariffs for PV systems below 10 kWp undercut the retail

electricity prices for households in Germany (Weniger et al, 2014) . With the increase of PV

feed-in tariffs and prices of grid electricity, using the PV generated electricity in the household

level is becoming more attractive than feeding it into the grid. PV systems with battery storage

can increase of self- consumed PV electricity. With a battery system, the excess PV electricity

during the day is buffered and later used at night. In this way, households equipped with a PV

battery system can reduce the energy drawn from the grid and therefore increase their self-

sufficiency.

2.1.4.Limitation of Electrical PV storage

Three barriers to a more widespread use of solar PV are that (i) electricity generation from this

source is limited to daytimes, (ii) depends on local weather conditions and (iii) fluctuates

strongly over the year. While the possibility of shifting the supply of electricity to different

times enhances the value of the electricity produced, adding storage technologies to a PV

system also raises the overall investment cost to be borne by plant operators. In Germany

programs now subsidize the use of storage technologies for residential PV. Considering the

falling costs for both PV and battery technologies, however, it remains controversial whether

and for how long these subsidies are necessary to drive the deployment of storage technologies.

The economic viability of storage is often based on an assumption that policy support in the

form of feed-in tariffs for solar photovoltaic power and/or additional premiums for self-

consumed electricity. However, feed-in tariffs in many countries have significantly decreased

over last years and are expected to be phased out. Therefore, it is important to investigate the

viability of storage in an environment without subsidies for PV and storage technologies.

Studies of the viability of storage for residential PV have usually investigated a limited number

of sizes for both the PV system and the battery storage. Without the assumption of no additional

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policy incentives, the chosen size of the PV system and battery storage strongly affect the

economic viability of the integrated PV and battery system. As a result, it currently remains

unclear when storage investments will be economically viable for a household that optimizes

the size of the PV system and the battery storage at the time of investment.

2.1.5.Economic impacts of performance for PV production and storage

PV energy worldwide from 2000 to 2010 grew from 1.5 GWp to 40 GWp cumulative capacity

respectively. In addition, system cost has reduced dramatically and the device efficiency has

increased steadily.

A novel method to obtain the optimum community energy storage (CES) systems for end user

applications (Parra et al, 2015) evaluates the optimum performance, levelised cost (LCOES), the

internal rate of return and the levelised value of suitable energy storage technologies. A

complimentary methodology was developed including three reference years (2012, 2020 and

zero carbon year) to show the evolution of the business case during the low carbon transition.

This method was applied with lead-acid (PbA) and lithium-ion battery (Li-ion) technologies

when performing PV energy time-shift using real demand data from a single home to a 100-

home community. The community approach reduced the LCOES to 0.30 £/kW h and 0.11 £/kW

h in 2020 and the zero carbon year respectively. These values meant a cost reduction by 37%

and 66% regarding a single home. PbA batteries needed from 1.5 to 2.5 times more capacity

than Li-ion to reduce the LCOES. The worst case scenario being for the smallest communities,

where the spiky demand profile required proportionately larger PbA battery capacities (Parra et

al, 2015).

To analyze a residential PV battery system in order to gain insights into their sizing a simulation

model was developed by Weniger et al (2014) and system simulations on a timescale of one

minute performed. In the considered long-term scenario the conjunction of PV systems with

batteries will be not only profitable but also the most economical solution (Weniger et al, 2014).

In 2014, Hoppmann et al. developed a techno-economic model that simulates the profitability of

battery storage from 2013 to 2022 under eight different scenarios for PV investment costs and

electricity prices in Germany, assuming no feed-in or self-consumption premium pay for

electricity generated using solar PV. For an economically-rational household, investments in

battery storage were profitable for small residential PV systems. The optimal PV system and

storage sizes rise significantly over time such that in their model households become net

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electricity producers between 2015 and 2021 if they are provided access to the electricity

wholesale market. Increases in retail or decreases in wholesale prices further contribute to the

economic viability of storage. Under a scenario where households are not allowed to sell excess

electricity on the wholesale market, the economic viability of storage for residential PV is

particularly high. Thus additional policy incentives to foster investments in battery storage for

residential PV in Germany were determined to be necessary only in the short-term. At the same

time, the increasing profitability of integrated PV-storage-systems may bring major challenges

for electric utilities that are likely to require increased investments in technical infrastructure

that supports electricity generation (Hoppmann, Volland, Schmidt, & Hoffmann, 2014).

To address how PV battery systems of various sizes could reduce the dependence of residential

customers on the central grid and their impact on CO2 emissions in United States, Hanser et al

(2017) analyzed how the costs of such systems change as customers attempt to decrease their

dependence on the grid, considering the installed cost of PV-battery systems and the cost of

electricity under a net-energy metered rate structure. The results showed that fully disconnecting

from the grid with a PV-battery system is impractical for most residential customers without

also having dispatchable backup generation (Hanser, Lueken, Gorman, & Mashal, 2017).

2.2. Photovoltaic Modules and Thermal Energy Storage (TES)

2.2.1.Use of latent heat storage for the thermal management of PV

In latent heat storage, thermal energy is stored/released by a material while changing its phase at

a constant temperature. It is also a pure physical process without any chemical reaction during

charge or discharge. The amount of heat stored is generally the latent heat of phase change

(latent heat of fusion for a solid-liquid transition and latent heat of vaporization for a liquid-

vapor transition), (Pelay et al, 2017).

Phase change materials (PCM) absorb thermal energy as latent heat at a constant phase change

temperature. PCM with a suitable phase transition temperature can be used to regulate the

temperature of PV cells (Huang et al., 2006a; Hasan, 2010b; Hasan et al., 2014a) thus

maintaining high efficiency for an extended period of time. Compared to other methods of

temperature regulation, the use of PCM has the added advantage of storing heat energy that can

be used asynchronously.

At initial heating, a PCM heats sensibly and when the PCM reaches the melting/solidification

temperature, the material absorbs latent heat, progressively melting. Melted PCM continues to

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warm further as it melts. The duration and temperature range over which the phase change takes

place depends on the mass of PCM and the thermal conductivity of PCM and any enhanced heat

transfer elements therein. Once PCM has completely changed phase the material will begin to

heat sensibly again. This process is illustrated in Figure 7 (Günther et al., 2009).

Fig. 10. Graphical representation of the variation of stored heat of a PCM with increasing temperature (Günther et al., 2009)

A classification of PCM in terms of melting temperature and melting enthalpy is shown in

Figure 8 (Cabeza et al., 2011). PCMs are divided into organic (paraffin, fatty acid), inorganic

(hydrated salts) and eutectic (mixture of organic or inorganic PCM) within the required melting

temperature of 20 ºC to 100 ºC. Cabeza el al. (Cabeza et al., 2011) divided PCM into groups;

Cooling applications up to 21ºC

22 ºC - 28 ºC for comfort in building applications

29 ºC - 60 ºC for hot water applications

High temperature applications requiring PCM of between 61 ºC - 120 ºC

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Fig. 8. PCM classification based on melting temperatures and melting enthalpy (Cabeza et al., 2011)

There has been considerable research on types of PCM, their applications and thermophysical

properties (Zalba et al., 2003; Sari et al., 2004; Sharma et al., 2009; Kousksou et al., 2014;

Yuan et al., 2014a). The advantages and disadvantages of PCM types for heat storage are

summarised in Table (Zalba et al., 2003).

Table 5. Advantages and disadvantages of organic and inorganic PCM for thermal storage (Zalba et al., 2003)

Organics InorganicsAdvantages Non corrosive

Low or no undercoolingChemical and thermal stability

Greater phase change enthalapy

Disadvantages

Lower phase change enthalapyLow thermal conductivityInflammability

UndercoolingCorrosivePhase segregationLack of thermal stability

The desired characteristics of a material are dependent on the application. However, PCM with

a high phase change enthalpy, being non-corrosive, with no subcooling and with chemical and

thermal stability would be an ideal choice but where this is unavailable material properties have

to be prioritised with regard to PV applications.

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PCM melting temperatures vary considerably due to material type, thermal properties and

chemical structure (Cabeza et al., 2011; Oró et al., 2013). PCM with specific melting

temperatures are available and their thermophysical properties known (Zalba et al., 2003;

Agyenim et al., 2010; Cabeza et al., 2011; Liu et al., 2012). Paraffin waxes are suited to lower

temperature applications (<100ºC) and molten salts can be used for high temperature

requirements.

2.2.2.Thermal Management of PV using PCM

In 1978, it was suggested PCM could act as a potential thermal storage if integrated with a PV.

This concept was patented in 1983 (Ames, 1983). However, after this, PCM was not

investigated as a means of cooling PV until the mid-1990s. There have been many experimental

and computational investigations into the use of PCM to regulate the temperature of PV (Ling et

al., 2014). The first study of PCM as a potential method to regulate the temperature of PV is

illustrated in Fig. 9 (Stultz and Wren, 1978).

Fig. 9 Phase change material integrated with a photovoltaic model (Stultz and Wren, 1978)

Stultz (Stultz and Wren, 1978) used Eicosane with a melting point of 36.7 ºC, with a Spectrolab

PV module used. The properties of Eicosane are detailed in Table. The high thermal expansion

is an undesirable property when integrated into a PV/PCM system.

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Table 6. Properties of Eicosane, the first PCM integrated with PV (Stultz and Wren, 1978) PropertyMelting temperature (ºC) 36.7Density: Solid (kg/m3) Liquid (kg/m3)

855.4778.5

Latent heat (kJ/kg) 246.5Specific heat: Solid (J/kg.K) Liquid (J/kg.K)

2.212.00

Expansion in phase change 15%Thermal conductivity W/(m2/K) 0.491

The experiment showed an increase of 1.4 % in the electrical efficiency of the PV. However, it

was noted this could be improved with enhanced thermal conductivity of PCM and thus heat

transfer from the PV to PCM. Stultz claimed if the PCM were successful in absorbing the

excess thermal energy, an improvement in power of 2 % to 3.5 % could be expected. A

PV/PCM system was not considered financially viable, however a PV combined with a thermal

storage system was seen to have the potential to be cost effective. This work was discussed in a

patent in 1983 but was not developed commercially (Ames, 1983).

In a patent published in 2005 (Al-Hallaj and Selman, 2005) PCM was incorporated in a battery

module system, including a fuel cell battery system. The invention refers to an arrangement

whereby the PCM absorbs a percentage of the heat generated upon a charge or discharge of

electrical energy as shown in Fig. 10. The objective of the system was to improve the power

supply of the system by regulating undesired temperature increases, minimise non-uniformity of

temperature and act as a heat sink.

20

Fig. 10. Configuration of electrochemical cell incorporated with PCM (Al-Hallaj and Selman, 2005)

Huang et al. (2000) progressed the concept independently with extensive experimental and

simulation studies. The first numerical PV/PCM model was validated successfully by

comparison with small scale experiments. Three systems were analysed under varying

environmental conditions; (i) an aluminium plate to simulate a PV cell; (ii) an aluminium plate

with a container filled with PCM to simulate a PV/PCM system; an aluminium plate with a

container filled with PCM with integrated fins, as in Fig. 1.

Temperature variation through the PCM and the solid–liquid transition of the PCM compared

well with predictions. Integrated PCM system with fins demonstrated effective temperature

regulation of the plate (Huang et al., 2004).

Fig. 11. Photographic image of melting front of PV/PCM system (Huang et al., 2004)

21

A 3D thermophysical model for a PV/PCM system based on the Navier-Stokes equation was

developed for predicting the convective velocities of melted PCM and temperatures inside the

PV/PCM system, this model was compared with a previously published 2D model. The 2D

model can be used to predict temperature of a simple linear PV/PCM system however the 3D

model allows the prediction of a line-axis system to be resolved (Huang et al., 2006b; Huang,

2007). The application of fins in a PV/PCM system in BIPV to improve low heat transfer rates

of PCM was found to moderate the increase in temperature of PV (Huang et al., 2006c).

Aluminium fins, integrated in a PV/PCM system, (Huang et al., 2011) were used to investigate

the melting process of PCM and the effect of differently spaced fins both theoretically and

experimentally using PCM RT27. Additional fins were shown to improve the temperature

control of the PV.

Hasan et al. (2010b) investigated experimentally the use of different PCM types to enhance the

thermal regulation of BIPV in a series of small scale indoor experiments. Five PCMs (RT20,

capric:palmitic acid, capric:lauric acid, calcium chloride and SP22) and four different systems

System A (Aluminium, internal width 5 cm), System B (Perspex, internal width 5 cm), System

C (Aluminium, internal width 3 cm) and System D (Perspex, Internal width 3 cm)) as in Fig. 12

were tested under three insolation values (500 W/m2, 750 W/m2 and 1000 W/m2) in a small

scale indoor experiment using a solar simulator to imitate solar conditions at an ambient

temperature of ± 1ºC.

Fig. 12. System A, B, C and D (Hasan, 2010b)

For insolation of 500 W/m2, System A with capric: lauric and capric: palmitic was shown to

maintain a lower PV temperature for the longest period, (up to 2.5 hours at 10 ºC lower than the

reference system). At 750W/m2 and 1000W/m2 as shown in Figure 13. System A with calcium

chloride was shown to maintain PV front surface temperatures 10 ºC below the reference for a

prolonged period of time compared to the other systems.

22

Fig. 13. Results from experiments at (a) 750 W/m2 and (b) 1000W/m2(Hasan, 2010b)

The thermal behaviour of the PV/PCM system was simulated for hot climatic conditions

(Cellura et al., 2008) with three paraffin waxes, that melted at 26 ºC - 28 ºC, 31 ºC, 28 ºC -

32ºC for PCM1, PCM2 and PCM3, respectively.

A proposed BIPV PV/PCM system using microencapsulated PCM (MEPCM) as shown in Fig.

14 (Ho et al., 2012), where navy is glass layer in PV, yellow is plastic layer in PV and grey is

PCM and each number represents a surface of the calculation model, each having its own

boundary conditions. The model showed an increase of 14 % in the PV/PCM system when

compared to a system without PCM, although this model had yet to be validated experimentally.

Fig. 14. Schematic of heat transfer through BIPV PV/PCM system (Ho et al., 2012)

A MEPCM with melting temperature of 26 ºC was simulated, during summer an increase of

0.13 % of electrical efficiency was shown with MEPCM compared to without MEPCM, winter

time confirmed an increase in efficiency of 0.42 %. The optimum aspect ratio (width/height) of 23

MEPCM incorporated in the system was recommended to be between 0.277 and 1. The melting

temperature of the PCM was above ambient and close to standard test conditions of the PV, this

was a small phase transition temperature which allowed for maximum transfer of latent heat to

PCM. At an ambient temperature of 26 ºC and solar irradiance of 600 W/m2 PCM with melting

temperatures of 26 ºC and 34ºC increase the efficiency of a PV module by 0.09 % and 0.12 %,

respectively (Ho et al., 2012). Corresponding results have been reported (Tanuwijava et al.,

2013). A similar study was undertaken to assess the performance of water-saturated

MEPCM/PV system, as in Fig. , using a CFD numerical simulation (Ho et al., 2013).

Fig. 18. Schematic of heat transfer in a PV/MEPCM module (Ho et al., 2013)

It was concluded that under summer conditions corresponding to an ambient temperature of 30

ºC and solar irradiance of 650 W/m2 that a PCM with a melting point of 30 ºC and 30 cm layer

of MEPCM be incorporated with the PV system. This melting temperature allows for phase

change to occur during peak sunshine hours and during night time the MEPCM could discharge

fully. Under these conditions an increase of 2 % in efficiency would be expected in the system

compared to a PV only. A numerical study of a water-surface floating PV/MEPCM system

showed a 1.8 ºC reduction in the temperature of the PV surface and 2.1 % increase in the

electrical output relative to a PV system without a MEPCM layer (Ho et al., 2015).

24

Sarwar et al. (2014) simulated the temperature regulation of the PV/PCM system illustrated in

Fig. 6 using a finite element numerical heat transfer simulation model that accounted the

electrical conversion of incident light and real-time heat loss boundary coefficients.

Fig. 16. Optics, electrical conversion and heat transfer mechanism in PV/PCM systems (Sarwar et al., 2014)

This model was validated by comparing the simulated and experimental temperature evolution

of PV at an incident irradiance of 1000 W/m2. The small-scale indoor experiment was carried

out on three inorganic PCM, capric-palmitic, capric-lauric and RT20. A 10 x 10 x 0.05 cm 3 PV

cell was encapsulated in 3 mm thick Perspex and attached to an aluminium container which held

the PCM. The model investigated the temperature evolution, velocities and phase change

fraction of the PCM and found ~1 ºC temperature difference between the modelled and

experimental results.

Jay et al. (2010), furthered investigated such systems via the experiment shown in Fig. 17 which

consisted of 4 panels, panel 1 and 2 with PCM of melting points 45 ºC and 27 ºC, respectively,

panel 3 is insulated from the rear and a panel 4 is a standalone PV. The PCM was encapsulated

into a honeycomb made of aluminium to promote heat conduction. Three solar irradiance

intensities of 600, 800 and 1000 W/m2 were simulated using a solar simulator and each system

characterised.

25

Fig. 17. (a) Aluminium honey comb (b) Experimental set up where (1) PV/PCM Tmelt 45ºC (2) PV/PCM Tmelt 27ºC (3) PV insulated and (4) PV standalone (Jay et al., 2010)

Under an irradiance of 800 W/m2, the temperature of the panels coupled with the PCM

remained below that of the reference panels for over 5.5 hours. The increase of instantaneous

electrical production due to the temperature regulation of the PV by the PCM was estimated to

be about 15 % – 23 % compared to a PV system alone. Simulations carried out using CFD

codes followed the same trends as the experimental investigations with small differences in the

predicted PV temperatures due to the approximation of material properties, heat transfer

coefficients, surface emissivity and air gap depth.

Japs et al. (2013), undertook an experimental analysis of a PV module integrated with a paraffin

based PCM. The PCM was incorporated with an aluminium-polymer compound to improve its

thermal properties. The PCM was shown to have a higher thermal conductivity but decreased

storage capacity when compared to its non-improved counterpart. Macro-encapsulated PCM

bags were attached to the back of a PV and were installed in Paderborn, Germany. Temperature,

current, voltage and MPP were recorded and it was found over a 25 day period the PCM

minimised daily temperature fluctuation of the surface of the PV compared to a system without

PCM. Also, the non-improved PCM provided lower operating temperatures of a PV module

initially although when melting occurs the improved PCM regulated the temperature for 5.5

hours compared to non-improved PCM.

Full scale outdoor experiments of the use of PCM to regulate the temperature of a PV system

were undertaken in Dublin, Ireland and Vehara, Pakistan (Hasan et al., 2014a). Three 65W PV,

as in Fig. 18, were used in the experiment where one served as a reference and the other two

were PV-PCM systems with a eutectic mixture of capric-palmitic and salt hydrate, respectively.

26

Fig. 18. Experimental set-up of PV-PCM systems, location of thermocouples and attachment of PV to PCM container (Hasan et al., 2014a)

Fig. 19. Surface temperatures of reference PV and PV-PCM systems measured in (a) Dublin and (b) Vehari (Hasan et al., 2014a)

The results were compared at peak instantaneous temperature of the reference PV.

Capric:palmitic PV-PCM system regulated the PV by 7 °C and the PV-PCM system with salt

hydrate maintained a temperature reduction of 10 °C compared to the reference PV as illustrated

in Fig. 19(a). This trend was also shown in Vehari, where capric:palmitic maintained a

temperature reduction of 10 °C and salt hydrate 21 °C when compared to the reference PV as

shown in Fig. 19(b).

Lo Brano et al. (2013; 2014) developed a one-dimensional finite difference model that

simulated the variable temperature profile of PV, the temperature change in the PCM and

melting fraction of the PCM. In the model first the temperature at each node was calculated then

27

the liquid fraction of the PCM. The simulation was validated using data derived from a full

scale experiment in Palermo, Italy illustrated in Fig. 0.

Fig. 20. Encapsulation of PCM and experimental installation (Lo Brano et al., 2013)

The phase change temperature of the PCM was 26 ºC – 28 ºC and experimental results were in

agreement with simulations. The investigation demonstrated that discharging at night is

essential to PCM performance. At night, it was found the PCM would not discharge fully as the

temperature of the PCM is higher than the solidification temperature therefore it was unable to

solidify so no latent heat storage capacity was available for the next day.

Using computational fluid dynamics (CFD) a 2D model was developed assessing the heat and

mass transfer of PCM located in the back of a PV panel (Biwole et al., 2013). An indoor

validation experiment was conducted using the set-up shown in Fig. 1.

28

Fig. 21. Experimental equipment (1) PCM, (2) heat exchangers, (3) PCM intake valve, (4) structure system, (5) flowmeter, (6) air intake valve, (7) metal stand and (8) insulation material (Biwole et al.,

2013)

The dimensions of the cavity containing the PCM was 167mm x 167mm x 30mm. Heat plates

were positioned on each side of the tank to simulate the heating from a solar panel. The PCM

used was RT25. The PCM was shown to maintain the PV temperature under 40 ºC for 80

minutes of constant exposure at 1000W/m2, compared to 5 minutes without PCM.

Browne et al. (2016) investigated a novel PV/T/PCM system which generated electricity, stores

heat and pre-heats water under the climatic conditions of Dublin, Ireland. The system combines

a PV module with a thermal collector; in which heat is removed from a heat exchanger embed-

ded in PCM through a thermosyphon flow. System performance was compared against a) the

same system without PCM, b) the same system without heat exchanger or PCM, and c) the PV

module alone as shown in Fig. 22. It was shown that the temperature attained by the water was

approximately 5.5 ºC higher when compared to a PV/T system with no PCM. PCM are shown

to be an effective means of storing heat for later heat removal in a PV/T system.

29

(a) (b)

(c) (d)Fig. 22 Schematic illustrating the design of (a) System 1(PV/T/PCM) (b) System 2 (PV/T) (c) System 3

(PV with container) and (d) System 4 (PV) in the experiment

An experimental study compared the performance of a Canadian Solar CS6P-M PV panel with

and without the PCM RT28HC (Stropnik et al. 2016). It was found the maximum experimental

temperature difference of PV and PV-PCM was 35.6 °C with an average increase in PV

efficiency of 1.7% as shown in (Fig. 23). The experimental data was used to validate a model

which showed an increase of 7.3% in power production of the PV/PCM system compared to

just the PV over a period of one year.

30

Fig. 23 Schematic of heat transfer in the PV-PCM panel

Preet et al., (2017), have investigated the thermal and electrical performance of the three

different systems, as shown in Fig. 24, which include a) a convectional PV panel, b) water

based photovoltaic/thermal system (PV/T) with double absorber plate and c) water based

photovoltaic/thermal system with PCM RT-20. The mass flow rate was varied and its effect on

the electrical and thermal efficiency was analysed. It was found that the inclusion of thermal

application in PV can provide a system with increased thermal and electrical output. The

maximum temperature reduction of 47% with water based PV/T and 53% with water based

PV/T-PCM has been achieved at mass flow rate of 0.031 kg/sec of water. It has been observed

that the Maximum increment in electrical efficiency was 10.66 with water based PV/T and 12.6

with water based PV/T-PCM (Preet et al,. 2017).

(a)

(b)

31

Fig. 24. Schematic illustrating the design of (a)Water based PV/T system (b) Water based PV/T-PCM

system

The integration of PCM has also been shown to act as a passive cooling method for

photovoltaics in the extremely hot and high insolation environment of United Arab Emirates

(UAE) (Hasan et al., 2017). A PV/PCM system was monitored for a one year period. Results

were used to validate a heat transfer model employing an enthalpy-based formulation. The PV-

PCM system enhanced the PV annual electrical energy yield by 5.9% in there climatic

conditions. Fig. 25, present the energy flow in the PV-PCM system.

Fig. 25. Schematic of

the heat transfer model of PV-PCM

32

1

22.12.2

2.2.1

2.2.22.2.3 Thermal Management of Concentrating PV (CPV) using PCM

PCM was first used in CPV in a patent in 1993, (Horne, 1993) where the concept of PCM to

protect PV cells in a solar concentrator from excessive temperatures was included. The selected

PCM melted at a temperature higher than the normal operating temperature range of the PV

cells, but less than the temperature at which the PV cells suffer thermal damage. Further to

work on asymmetric compound parabolic photovoltaic concentrators (ACPPVC) (Mallick et al,

2004, Wu, 2009; Wu and Eames, 2011) which concentrate significant diffuse solar radiation,

Wu et al. designed and constructed an ACPPVC integrating PCM RT27 to the back, as in Error:

Reference source not found26, to regulate the temperature. The system had a concentration ratio

of 2 (Wu, 2009).

(a) (b)

Fig. 26. ACPPVC integrated with PCM (a) schematic and (b) photo (Wu, 2009)

At an incident solar irradiance of 672 W/m2 and incident angle of 0 º the temperature of the

solar cells of the ACPPVC-55/ PCM was reduced by 18 ºC during the phase change compared

to ACPPVC-55 system. The predicted electrical conversion efficiency of the ACPPVC-55/PCM

33

is 10 %. Further tests under various incident angles and solar irradiance intensities found that at

solar irradiance intensities of 280 W/m2 the PV temperature was reduced by 7ºC for

approximately 10 hours when using RT27 with an increase of 5 % in the electrical conversion

compared to ACPPVC-55.

A V-trough CPV system was investigated where a metal-wax composite PCM was used to

regulate PV temperature (Maiti et al., 2011), illustrated in 27. Paraffin wax with a phase change

range of 56 ºC - 58 ºC was located on the rear of the module. To enhance low thermal

conduction in the PCM, aluminium lathe turnings were embedded. Indoor experiments

maintained the temperature between 65 ºC - 68ºC for 3 hours at 2300 W/m2 generated using a

solar simulator. The temperature of the PV rose to 90ºC in absence of the PCM. Outdoor

experiments showed maximum temperatures of between 78 ºC – 80 ºC in the absence of PCM

and 64 ºC - 65 ºC with the PCM as a temperature regulation mechanism though this was at a

lower solar irradiance intensity of 1982 W/m2 generated by the initial radiation being

concentrated in the V-trough.

Fig. 27. V-trough PV system with integrated metal-wax composite PCM (Maiti et al., 2011)

PCM was further investigated theoretically in a CPV system with concentration of 2 suns and

was found to be a technically feasible option for suitable sites with high solar concentration

34

(Lillo-Bravo et al., 2011). Twelve sites were chosen for which a CPV/PCM model was

simulated under the environmental conditions of each site. Four phase change temperatures

were tested, 25 ºC, 35 ºC, 45 ºC, and 55 ºC. It was found that a concentrated system located in

Cairo generated 37.2 % more electrical power than a system without PCM, followed closely by

Cairo and London produced the least increase in energy of 18 %. The improvement in

efficiency was closely linked with the location of installation and the possibility of solar

concentration. It should be noted that this model has not yet been validated.

A hybrid CPV-thermoelectric concept integrated PCM as a store of heat generated by a

concentrator as in Fig. 28 (Li,. Y, et al., 2013). A series of calculations based on conversion

efficiencies of the PV cell and thermoelectric generator suggest system efficiency improved by

30 % when a high-grade cold energy storage system was added.

Fig. 28. Schematic of PV-thermoelectric power system with the use of PCM as TES (Li et al., 2013)

A two-axis concentrating photovoltaic system thermally regulated by PCM was fabricated and

tested outdoors in Pakistan as in Fig. 29 (Sarwar, 2012).

35

Fig. 29. Concentrating PV system with PCM and fins for cooling (Sarwar, 2012)

Numerous PCM were tested and it was found that selection of the optimum PCM depends on

application. Lauric acid was found to reduce the peak PV temperature by 22 ºC and palmitic

found a maximum temperature difference of 19.5 ºC. However, palmitic has a higher heat of

fusion and temperature regulation capabilities.

2.2.4 Hybrid Photovoltaic/Thermal/Phase Change Material Systems

A photovoltaic/thermal (PV/T) system converts solar radiation into electrical and thermal

energy. The incorporation of thermal collectors with PV technology can increase the overall

efficiency of a PV system as thermal energy is produced as a by-product of the production of

electrical energy. Passive cooling is a buoyancy-driven and the use of an external mechanical

system is known as active or forced cooling. The removal of heat from the PV occurs

immediately once a temperature differential develops between the PV and the transfer fluid.

There are two options for disposal of excess thermal energy collected from the PV; transfer of

heat to air or water. The pre-heated fluid is diverted directly to an end application such as warm

water or air which can be used for purposes such a space heating or domestic hot water

requirements.

Several review papers have been published in the use of PCM as a thermal management

technique of PV, emphasising the growing body of literature in the area (Browne et al., 2015a;

Ma et al., 2015). Yin et al. 2013, have presented a PV/T system with integrated heat storage

using PCM as shown in Error: Reference source not found30. Water is directly heated by

36

thermal energy from the PV; it flows via a thermosyphon through the PCM which will store the

heat from the water for later applications.

Fig. 30. Cross section of residential PV/T system (Yin et al., 2013)

The water reached a temperature which was found to be useful in domestic applications. In

ambient temperatures heat captured by the PCM prevented it entering the domestic dwelling and

significantly reducing the cooling load.

A one-dimensional energy balance model of a PV/T/PCM (PV/Thermal/PCM) system was

simulated as shown in Error: Reference source not found31 (Malvi et al., 2011). The model is

divided up into a series of nodes; at each of which a temperature is calculated. Investigations

into water flow rate, PCM thickness, PCM melting point and PCM conductance were carried

out using the model. The most effective flow rate for PV cooling is approximately 10 l/h;

however, this is undesired for water heating applications as the water outlet temperature is

reduced. The optimum thickness of PCM was found to be 0.03 m which increases the PV

efficiency by approximately 6.5 %. The temperature range changes throughout the year which

emphasises the issue of having to change the melting temperature of the PCM throughout the

year. The PV output increases by 3 % if the conductance of the PCM is increased by a factor of

10. New PCM with improved thermal conductivities would be required. The electrical output of

37

the PV/T/PCM system was shown to increase by 9 % when compared to a PV system only

(Malvi et al., 2011).

Fig. 31. Schematic of simulated system (Malvi et al., 2011)

The addition of PCM in PV/PCM systems has been shown to lower the operating temperature of

PV by removing excess heat (Malvi et al., 2011).

A numerical simulation of BIPV/T-PCM vented and un-vented Trombe-Michel walls,

illustrated in Error: Reference source not found32, was validated via comparison with the

experimental results from a BIPV/T system in this case ventilation wall is part of PV/T collector

system (Aelenei et al., 2013). The air cavity allows for ventilation and each system has been

investigated with and without it. The results of non-ventilation showed the PCM caused a

decrease of 7 ºC when comparing the temperature of the air cavity due to the latent heat storage

of the PCM. However, ventilation of the systems showed a much lower difference of 2 ºC

(BIPV/T-PCM 28 ºC and BIPV/T 30 ºC) suggesting that heat is removed by ventilation rather

than to the PCM.

38

Fig. 32. Simulated BIPV/T system with integrated PCM wallboard (Aelenei et al., 2013)

The investigations show that the effect of the storage in the PCM reduces the PV temperature

and thus raises the PV efficiency. However, during winter conditions the storage of heat causes

adverse effects as the heat transfer from air cavity to the interior of the room is reduced (Aelenei

et al., 2013). In a numerical and experimental study of this system the maximum electrical and

thermal efficiencies reached were 10 % and 12 %, respectively (Aelenei et al., 2014).

2.2.5 Use of Sensible Heat Storage in PV applications

In sensible heat storage, thermal energy is stored/released by raising/decreasing the temperature of a storage material. It is a pure physical process without any phase change during charge or discharge. Therefore, the amount of heat stored depends on the product of the mass, specific heat, and temperature variation of the storage material. In addition to the density and the specific heat of the storage material, other properties are also important for sensible heat storage: operating temperature, thermal conductivity and diffusivity, chemical and thermo-chemical stability of the materials and the cost, (Gil et al., 2010).

39

The incorporation of PV modules in a building can create additional functionality for the PV as

wall and roofing elements, canopies and/or shading devices that replace “conventional” building

materials; this is referred to as building integrated PV (BIPV).

Elevated operating temperatures arise as buildings can thermally insulate incorporated PV. A

full-scale assessment of BIPV modular temperature for six different types of PV has been

analysed by Maturi et al. (Maturi, Belluardo, Moser, & Del Buono, 2014). It was found that

copper indium gallium selenide (CIGS) PV operate at a temperature 8ºC higher than the

operating temperature of mono crystalline potentially affecting the efficiency by 3%, while the

environmental conditions was kept constant at 1000W/m2. The thermal behavior of various PV

facades have been investigated by (Krauter et al., 1999). The experimental results showed that

the thermal-insulating building facade increased PV cell temperature by 20.7K causing a 9.3%

loss of electrical yield compared to an actively cooled PV cell. Another solution to limit the rise

of the temperature of system is to combine with a solar water heating collector and PV cells

using photovoltaics/thermal (PV/T) collector. Therefore, the recovered energy (heat) can be

used for the heating system as well as for the domestic hot water by a flow of glycol water at the

back side of PV modules (poly-crystalline in our study). The annual photovoltaic cell efficiency

for Mâcon, France, showed a BIPV system to operate a cell efficiency of 6.8%, which is

equivalent to a 28% lower efficiency than to a non-integrated PV system, (Fraisse, Ménézo, &

Johannes, 2007).

In BIPV/thermal (BIPV/T) systems heat is extracted from behind the PV panel through a

convective air or water flow that can be forced or natural. The behavior of forced air circulation

and natural convection were numerically designed for a BIPV installation on a hotel building in

southern China (Chow, Hand, & Strachan, 2003). The results confirm that the difference in

electricity output between using forced or natural convection was less than 1% annually. It was

also shown that natural convection demonstrated better in subtropical climates due to the

effectiveness in space heat gain reduction compared to BIPV systems. The application of using

fins and a thin metallic sheet midway between the PV and wall can enhance the overall

performance and heat transfer from the PV to the air circulating in a photovoltaic/thermal

(PV/T) solar collector (Tonui & Tripanagnostopoulos, 2007).

The development of the performance of a photovoltaic water pumping system using spraying

the water over the photovoltaic cells was investigated by (Abdolzadeh & Ameri, 2009). This

was achieved by flowing water to the front or back surface of a PV in order to remove the heat

40

from the system. It was found that the temperature of the PV was reduced by 23% when water

was sprayed over the front surface of the PV, while giving an increase in the mean PV cell

efficiency of 3.26% (Abdolzadeh & Ameri, 2009).

Zondag et al.(Zondag, de Vries, van Helden, van Zolingen, & van Steenhoven, 2003) studied

the performance of four groups of PV/T systems of nine different designs; (a) sheet-and-tube

PVT collectors, (b) channel PV/T collectors (c) free flow PV/T collectors and (d) two absorber

PV/T-collectors. The comparison of the thermal and electrical efficiencies is presented in

addition to the yearly yield of hot water. It was found that a sheet-and-tube PV/T collectors

design was the best option due to the feasibility of manufacturing process. However, it was not

the most efficient when compared to the other systems. Wu et al. (Wu, Zhang, Xiao, & Guo,

2011), modelling a PV/T system using water as the transfer fluid, found that the thermal and

electrical efficiencies could reach 63.65% and 8.45% respectively.

2.2.6 Use of Thermochemical energy storage

Thermochemical energy storage is not usually used along with PV systems but rather with

concentrated solar power (CSP) plants. CSP plants are generally coupled with conventional

fuels and utilize thermal energy storage (TES) to overcome the intermittency of solar energy.

Different from sensible or latent heat storages, thermochemical TES technology is based on

reversible chemical reactions, which are characterized by a change in the molecular

configuration of the reactants (combination or decomposition). Solar heat is used to drive an

endothermic chemical reaction, and then stored in the form of chemical potential. During the

discharge, the stored heat can be recovered by the reversed exothermic reaction, sometimes by

adding a catalyst, Pelay et al, 2017.

The advantages of thermochemical storage rely on their high energy density (up to 10 times greater than latent storage and the indefinitely long storage duration at ambient temperature. As a result, it is a very attractive option and fairly economic competitive Metallic hydrides, carbonates system, hydroxides system, redox system, ammonia system and organic system can be used for thermo- chemical heat storage at medium or high temperatures (300–1000 °C), Pardo et al., 2014.

Beyond choosing the suitable TES technology for CSP application, the TES system must be

coupled in a proper way with the power generating cycle (e.g., Rankine cycle). Pertinent

41

concepts that integrate TES system and the power generating cycle remain as one of the key

issues for the actual application of TES technology, Pelay et al., 2017.

3 Integration of PV- Energy Storage in buildings

The solar thermal energy stored in the PCM in the BIPV can provide a heating source for the

Heat Pump (HP) to provide high temperature heat for domestic heat supply. Underfloor heating

is an efficient and economical method for home heating compared which can use the low

temperature heat supply from HPs. Research on using heat pump with integrated phase change

materials layer for underfloor heating has shown that this can save operating cost and improve

the thermal comfort (Huang and Hewitt, 2015). A research of collecting low temperature heat

from BIPV-PCM for HP evaporator heat supply and then providing high temperature heat for

domestic heat supply is carried out by (Huang,.2016). The supplied heat by HP is used for PCM

layered underfloor heating system. The schematic diagram in Fig. 33 shows the process of

extracting solar heat from BIPV-PCM through the cycling cupper pipes and used for underfloor

heating system (Huang, 2016). HP can extract the low temperature solar thermal energy stored

in the BIPV-PCM system and provide high temperature hot water for underfloor heating system

in the residential buildings. PCMs incorporated into solar energy thermal storage or underfloor

heating system in buildings may be suitable for absorbing solar energy directly or store the heat

from HP during off peak time. One of the main barriers for this application is how to improve

the low thermal conductivity of the PCM in order to achieve a quick thermal response with

longer thermal store performance in the buildings. Therefore, a comprehensive system has been

investigated to use PCMs for solar energy storage in the domestic buildings (Huang, 2016).

42

BIPV-PCM

Evaporator

Condenser

2

1

3

4

Heat Pump

Underfloor heating

Heat supply

Heat extraction

Compressor

Fig. 33. A schematic diagram of using HP for supplying heat for PCM layered underfloor heating system and extracting solar heat from BIPV-PCM through the cycling cupper pipes (Huang, 2016)

In order to compare the effect of PV temperature regulation and the energy management

capacity, the BIPV-PCM-HP system the three cases shown in Fig. 34 have been studied.

PCM 1

PCM 1

PCM 2

PCM 2

PCM 2

D

E

C

PCM 1

PCM 1

PCM 2

PCM 2

PCM 2

D

E

C

PCM 1

PCM 1

PCM 2

PCM 2

PCM 2

D

E

C

Case A Case B Case C

Fig. 34. Three BIPV-PCM-HP system operation cases for simulation (Case A: RT27 PCM only with metal fins; Case B: Augmented copper pipes in the PCM without cycle; Case C: Augmented copper pipes

with fluid cycle from HP) (Huang, 2016)

43

A numerical simulation model has been validated and used to analysis the thermal performance

of PCM layered underfloor heating under difference heating modes. Different layout of the

under-floor heating pipes with PCMs as floor mass material were analysed with dissipated heat

from BIPV for indoor environment heating process. The thermal performance of heating

system under realistic diurnal temperature boundary conditions was predicted. An illustrative

example of this analysis is shown in Figure 35.

Fig. 35. Predicted thermal isotherms for the cross-section of floor structure of 70 mm concrete filling and 200 mm pipe spacing under 40 °C water at heating process (Huang and Hewitt, 2015)

4 The potential and the Role of Energy Storage for PV and future energy development

Incentives from supporting policies, such as feed-in-tariff and net-metering, act will gradually

phase out with rapid increase installation decreasing cost of PV modules and the PV

intermittency problem. The PV self-consumption becomes more attractive because the self-

consumed electricity generally has more economic values than the exported electricity (Castillo-

Cagigal et al., 2011; Luthander et al., 2015). Under the existence of intermittent solar resource,

Electrical energy storage (EES) can continue to maintain the stability of the power grid in an

effective and economically feasible manner. Further investigation with detailed analysis and

optimal solutions are needed to simultaneously determine the energy storage method, the

storage capacity and the operation strategy.

4.1 Potential demanding of Energy storage on Electric Vehicles

China is to set a deadline for the end of sales of conventional internal combustion engine

vehicles to accelerate the move towards greater electric vehicle (EV) use in the country. The

sales of “new energy vehicles” (fully electric or plug-in hybrid EVs) should reach 2mn by 2020

44

and account for over 20% of total vehicle production and sales by 2025. The UK and France in

July 2017 both countries will ban conventional passenger vehicles by 2040. India has also

announced that it is looking to ban conventional vehicles by 2030. This rapid move to electric

vehicles (EVs) needs to accelerate the infrastructure and support which will allow the rollout to

go further and faster. However, the full integration of electric vehicles into energy infrastructure

is a challenge. This will leave huge potential market on the PV integrated cars with energy

storage capacity (BBC report, 2017).

4.2 Energy storage for large-scale PV system

A forecast of global PV generation shown in figure 36 (IEA, 2014) predicts a sharp growth in

PV capacity with PV providing 16% of global electricity by 2050. Such an increase will bring

economic and technical challenges to integrate solar power into the grid due to the diurnal and

stochastic nature of solar energy. Electrical energy storage (EES) have explored in

improvements and services to power systems, however more work needs to be done in smart

grid as a key component in modern power systems developments for secure and reliable

operation. Although the EES market is expanding in, there is a lack of standards and methods

for comparing the efficiency and performance of EES in a PV system.

Fig.36. Trends for global power generation of solar PV system and share of total electricity generation [IEA, 2014]

45

4.2.1 Technical standards for PV system performance

Some technical standards for PV system performance, design requirements and capabilities

have been created, however interconnection policies that consider the benefits of both

distributed generation owners/operators, and distribution companies are required to address

major problems such as communication, control, short-circuit protection, electrical isolation and

surge protection (Lai, et al., 2017). In the future, the standards for EES in the PV system need to

be defined.

4.2.2 Stability issues

Large scale PV generation can reduce generation cost in the industry and could avoid the effect

of uncertain carbon pricing policies and non-deterministic future fossil fuel prices, but it has

issues with the cost related to creating surplus energy either storing it or transmitting it to the

external grid. The high uncertainty and fluctuation from the PV power cause poor power

quality, Volt/VAr control issues. The stability problems and control methods of PV generators

for secure and reliable operation are the main focus in many research work. The pinning reserve

and dispatching strategies for energy storage could play important part in the stability of future

power system with high PV penetration (Shah et al., 2015). (Tamimi et al., 2013) investigate the

system stability at different PV penetration levels at up to 2GW on three cases: the centralised

farms with voltage regulation capacities, centralised farms without voltage regulation capacities

and the distributed units. It concludes distributed solar PV generators are superior than

centralised solar PV generators, i.e. solar farms. Although some work has been carried out on

analysis of the transient stability and small signal stability for power system, additional research

needs to be done to assess with different size and capacities of the PV power generation system

with different penetration levels.

The system stability improvement has also been studied on a 10 MW residential PV system by

using methods to reduce the fluctuation in the power generation (Omran et al., 2011), (1) EES

utilisation; (2) dump loads utilisation; and (3) PV power curtailment. The consequence with PV

output power stability improvement is a revenue loss. The combine use of EES and power

curtailment is found to be the most economical solution.

4.3 Dynamic ambient forecasting for PV generation and EES

PV performance can be highly influenced by external factors such as ambient temperature, the

cloud shape, density, size and the speed of cloud cover (Katiraei and Aguero, 2011). Incident 46

solar radiation and ambient conditions forecasting accuracy has a tremendous influence to the

PV and EES systems planning and operation. The accurate assessment of solar irradiance

available at a particular time instance is not deterministic problem due to the intermittent and

dynamic nature of solar resource. Therefore, forecasting and prediction methods are needed to

be developed to provide better estimate of the solar irradiance resource and surrounded ambient

conditions.

A comprehensive review of the theoretical forecasting methodologies for both solar resource

and PV power has provided by Wan et al., 2015. A number of forecasting models have been

developed for solar resources and power output of PV plants at utility scale level in the past few

years, but due to the chaotic nature of weather systems and the uncertainties involved in

atmospheric conditions such as temperature, cloud amount, dust and relative humidity, precise

solar power forecasting can be extremely difficult. Forecasting models of PV power system are

divided in to four classes: statistical models, AI-based models, physical models and hybrid

models. Statistical approaches are based on historical data which is used as inputs for the

statistical model and the variable to be predicted. A novel multi-time scale data-driven forecast

model based on spatio-temporal (ST) and autoregressive with exogenous input (ARX) is

developed for a solar irradiance forecast model. Simulation results that use the real solar data of

PV sites in California and Colorado demonstrate the proposed model can derive satisfactory

results for 1 h and 2 h look-ahead times (Yang et. al., 2015). AI techniques due to the high

leaning and regression capabilities, have been widely employed for modelling and prediction of

solar energy. Physical models utilize solar and PV models to generate solar irradiance/power

prediction. The generalized framework of physical approaches is shown in Fig. 37. In practice,

various hybrid solar prediction methodologies can be proposed to integrate the merits of

different types of prediction models which can offer forecasting of hourly global horizontal

solar radiation and forecasting of a high resolution solar radiation database.

47

Fig. 37. Typical framework of physical approaches for PV power forecasting.

4.4 Summary of future energy development

It has become a major point of interest and possible bone of contention that in a future near-

decarbonised electricity network i.e. 80% reduction in emissions by 2050, will enable a low

carbon space heating market through the deployment of electrically-driven heat pumps. It is

recognised that such a vision will place a significant extra burden on the existing electricity

network. This will be especially true of the low voltage network at Distribution System

Operator level e.g. 11kV or below but as such systems do not operate in isolation, challenges

will also be seen at the Transmission Operator level as well. This may be further complicated by

the perceived rapid deployment of Electric Vehicles well before 2050.

Demand Side Management in its many shapes and forms becomes part of the solution. The

Thermal Mass of Buildings can be deployed to understand duration of off-times after a period of

heating while maintaining thermal comfort. Understanding End User Behaviour is critically

important in this process as it is believed that the smart learning of the behaviour of the building

can be subtly tailored to end-user needs. Selective fabric retrofits and selective occupancy

controls can adjust these parameters further giving an element of control to the end-user while

satisfying electricity network operator needs.

The deployment of energy storage is also an option. Heat Pumps and Water Storage (or sensible

heat) trials and experiments reveal both noticeable heat losses from the tanks and large sizes of

tanks for significant scales of time management. Finding the space for larger tanks in typical

UK housing types would be challenging. However, aggregation of large numbers of these units

that are electricity network controlled (at the distribution operator level for example) has the

48

potential to reduce individual storage volumes thus potentially realising greater social

acceptance. Novel materials e.g. phase change or thermochemical will deliver further reductions

in size and low temperature thermal networks may upgrade heat pump performance and

increase energy efficiency. Hybrid Heat Pumps or gas driven types (adsorption or absorption)

will utilise existing gas networks that may be increasingly decarbonised with biogas or

hydrogen deployments.

5. Conclusion

Domestic and local Photovoltaic (PV) system deployment would provide electricity network

relief when accompanied by energy storage. Thermal storage will satisfy thermal comfort needs

when operating with heat pumps while battery deployment may operate with electric vehicles

and/or electric heat pumps at times of local grid congestion. Such batteries and electric vehicles

will also become demand side response units in their own right, the latter whether you are at

home or not, with Vehicle-to-Grid technology.

The combinations of such technologies will address many of the emerging low voltage network

problems as we attempt to decarbonise our energy use. The low voltage network has become the

focus of such challenges but sees little investment. The rates of penetration of the technologies

is unknown and their performance under real end-use conditions is also less well understood.

The electricity network investment avoidance costs are not yet proven.

Acknowledgements

The authors would like to acknowledge the European Union’s Horizon 2020 research and

innovation programme under grant agreement No 657466 (INPATH-TES) and the ERC starter

grant (6XXXX). Further to this they would like to acknowledge support from Science

Foundation Ireland.

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